As a head-up display (HUD) device, for example, one disclosed in Patent Literature 1 is known. The HUD device is provided with a display unit for displaying an image, and a combiner that can magnify and display the image, and can transmit a scene (background) in front of a vehicle, and is configured to allow a user to see display information such as a vehicle speed and mileage being superimposed on the front scene. The HUD device is further provided with an optical sensor for detecting background brightness for adjusting display brightness of the display unit in accordance with ambient brightness.